Phenyl substituted Mg porphyrazines: The effect of annulation of a chalcogen-containing heterocycle on the spectral-luminescent properties

We have performed complex experimental and theoretical investigations of the spectral-luminescent properties and electronic structure of new phthalocyanine analogs, Mg octaphenylporphyrazine and its derivatives with an annulated thiadiazole or selenadiazole ring instead of two phenyl groups. Fluorescence characteristics have been determined at 293 and 77 K: emission, excitation, and fluorescence polarization spectra; fluorescence quantum yield ?? _F , and lifetime ?? _F . Annulation of a five-membered chalcogen-containing heterocycle leads to splitting of the long-wavelength absorption band Q(0-0) and to the bathochromic shift of its longest wavelength component Q _x (0-0), which increase upon passage from S to Se. At the same time, the fluorescence quantum yield ?? _F and lifetime ?? _F decrease, which is related to the intramolecular heavy-atom effect. The geometric structure of the ground state of the Mg porphyrazine molecules has been determined based on the density functional theory (DFT), and excited electronic states have been calculated with modified parametrization of the INDO/S method, INDO/Sm. Semiquantitatively, the calculated level positions of the lowest Q states and spectral shifts of Mg octaphenylporphyrazine and S-derivative agree with experimental data. For the range of the Soret band, calculated transition energies and their intensity distributions substantially depend on the dihedral angle ?? between a phenyl ring and porphyrazine macrocycle. We show that, based on calculations at the angle ?? = 60°, bands in the observed absorption spectra can be assigned with an accuracy of ??2000 cm^?1.     

Numerical simulation of nonlinear properties of fullerene-containing one-dimensional photonic crystal

A model of interaction between a one-dimensional photonic crystal (PC) containing a defect (nonlinear optical layer of C₆₀ fullerene) and radiation with a wavelength ??₀= 1064 nm under steady-state conditions has been considered. This structure is a Fabry-Perot microcavity: a fullerene layer (with a thickness multiple of ??₀/2) is placed between the interference mirrors formed by alternating layers ??₀/4 thick. The PC under consideration (1) has a narrow transmission band in the vicinity of ??₀ against the background of a relatively wide 100%-reflection band (photonic band gap) in the linear mode and (2) provides multiple amplification of the radiation intensity in the intermediate layer with respect to the external radiation intensity. Since C₆₀ fullerene exhibits a significant optical Kerr nonlinearity, the optical thickness of the intermediate layer under irradiation deviates from the value multiple of ??₀/2; as a result, 100% transmission for ??₀ changes to almost 100% reflection at a certain radiation intensity. Thus, this structure behaves as a peculiar optical limiter.     

Two-photon cross-section measurement of meso-tetra-hydroxyphenyl-chlorin using femtosecond laser pulses

Experiments were carried out using the femtosecond laser to compare the fluorescence amplitude produced by two-photon excitation at 800 nm and 400 nm (i.e. to the strong absorption band in mTHPC). Using an experimental arrangement where the fluorescence sampling geometry was kept unchanged between the two exposure wavelengths a value for the two-photon cross-section of mTHPC at 800 nm of ?1.8 × 10^?57 m⁴ s/molecule/photon was derived. This is larger than the value estimated using a simple theoretical model, probably because the calculation neglects ??resonant?? assistance provided by the real state at ??650 nm (S ₀). In terms of order of magnitude the values are in agreement with the two-photon cross-section for other organic species.     

Magnetooptics of magnetic-dipole transitions in the rare-earth paramagnetic garnets

The Faraday effect (FE) of the rare-earth (RE) paramagnetic garnet crystals associated with magnetic-dipole (MD) transitions has been investigated theoretically over a wide temperature range 80 to 300 K. Over this temperature range, the ??gyromagnetic?? Verdet V _gm ^(m) of the Eu₃Ga₅O₁₂ (EuGG) is not a linear function of the magnetic susceptibility ?? measured along the [111] axis. We find that the nontrivial character of the temperature dependence of the MD Verdet constant V _gm ^(m) is associated with the presence of a contribution to the Faraday rotation of the Van Vleck ??mixing?? by an external magnetic field of the Eu³⁺ electronic states. The optical absorption and magnetic circular dichroism (MCD) of Eu³⁺ ion in EuGG at 85 K for the MD transition ⁷ F ₀ ?? ⁵ D ₁ were measured along the [111] axis. We show that the appearance of the MCD feature observed for the absorption line near 526.6 nm for the ⁷ F ₀ ?? ⁵ D ₁ MD absorption band may be well-explained by the contribution of Van Vleck ??mixing?? between the Stark singlet states from the ⁷ F ₀ and ⁷ F ₁ multiplet manifolds mixed by the external magnetic field H.     

On the possibility of applying a mirror ellipsoid of revolution to determining optical properties of biological tissues

Possibilities of a measuring system that uses an ellipsoidal mirror for determining the coefficients of scattering ?? _s and absorption ?? _a , as well as the anisotropy factor g, of a biological tissue are studied. Based on the measured values of diffuse reflection and total and collimated transmission and on a simulation by the inverse Monte Carlo method, the values of optical parameters of porcine epidermis were obtained in vitro at a wavelength of 632.8 nm.     

Effect of small magnetic corrections to susceptibility on the angular dependences of the reflectivity of polarized X-rays from multilayer structures

The X-ray resonant magnetic scattering (XRMS) method allows for the determination of optical constants including magnetic corrections, which are significant near the atomic X-ray absorption edges, by the shift of the Bragg angle of the reflection from periodic multilayers. Recently, Valvidares et al. [Phys. Rev. B 78, 064406 (2008)] revealed significant differences in the shape of “magnetic” Bragg reflection peaks from a [Co₇₃Si₂₇(50 Å)/Si(30 Å)]₁₀ film for two opposite states of antiferromagnetic interlayer ordering. Valvidares et al. assumed that these features can be explained by the presence of the reflection-induced magnetic resonance correction. We have demonstrated that such corrections in the case of antiferromagnetic structures do not lead to a shift of the Bragg peak, but the shape of magnetic peaks is explained by the interference of the magnetic and nonmagnetic reflection amplitudes.     

Beats of quantum oscillations of the hall coefficient and resistivity in semiconductor alloys n-Bi0.93Sb0.07 at a small deviation of the magnetic field direction from the trigonal axis

Quantum oscillations of the resistivity ??₂₂ and Hall coefficient R _12.3 in the semiconductor alloy n-Bi_0.93Sb_0.07 have been studied at H ?? C ₃ and j ?? C ₁ in magnetic fields to 14 T and at temperatures of 1.5, 4.5, 10, and 20 K. At temperatures of 1.5 and 4.5 K, beats of quantum oscillations of ??₂₂ and R _12.3 due to a small deviation of the magnetic field H from the crystallographic C ₃ axis have been observed. To determine the oscillation period ?? _i , cyclotron mass m _ci , cyclotron frequency ?? _ci , and extreme section S _extri, experimentally measured quantum oscillation beats have been compared with the model beats of oscillations of three harmonic functions, two of which have close frequencies. The deviation of the parameters ?? _i , m _ci , and S _extri from the same parameters when the magnetic field H exactly coincides with the trigonal C ₃ axis has made it possible to estimate the magnetic field H deflection angle from the trigonal C ₃ axis, which is ??1°.     

Superconducting properties of tin embedded in nanometer-sized pores of glass

The electrical resistance of tin embedded from a melt in porous glasses with an average pore diameter of ??7 nm has been investigated at low temperatures in magnetic fields up to 2 T. The temperatures of the transition to the superconducting state for nanocrystalline tin have been determined in magnetic fields of 0, 0.3, 0.5, 1.0, 1.5, and 2.0 T. It has been found that the temperature and magnetic-field dependences of the electrical resistance of the nanocomposite under investigation exhibit two transitions to the superconducting state. The nature of the double superconducting transitions has been discussed. The H _c -T _c phase diagram has been constructed using the entire set of data on the magnetic-field and temperature dependences of the electrical resistance of nanostructured tin. This phase diagram indicates that the upper critical magnetic field H _c2(0) for nanostructured tin is almost two orders of magnitude higher than the corresponding field for bulk tin.     

Spectral characteristics and fluorescence quenching of N,N′-bis(4-pyridyl)-3,4:9,10-perylenebis(dicarboximide) (BPPD)

The photophysical properties such as electronic absorption, molar absorptivity, emission spectra, fluorescence quantum yield and fluorescence lifetime of N,N′-bis(4-pyridyl)-3,4:9,10-perylene bis(dicarboximide) (BPPD) have been measured in different solvents. Both electronic absorption and fluorescence spectra are not sensitive to medium polarity, while the fluorescence quantum yield (?_f) is solvent dependent. The ground state geometry has been computed by using density functional theory (DFT), the transition from HOMO to LUMO from perylene core with maximum absorption at 512 nm and HOMO–LUMO energy difference equal 2.53 eV. BPPD dye undergoes molecular aggregation to dimmer or higher aggregates in dimethyl sulfoxide (DMSO). Crystalline solids of BPPD gives excimer-like emission at 676 nm. The fluorescence quenching of BPPD is also studied using hydrated ferric oxide nanoparticle (FeOOH), and the Stern–Volmer rate constants (K_sv) were calculated as 8×10⁶ and 9.2×10⁶ M^?1 in ethanol and ethylene glycol, respectively.     

Photodynamics of intramolecular proton transfer in polar and nonpolar biflavonoid solutions

Using methods of steady state luminescence and femtosecond spectroscopy, we have studied the mechanism of intramolecular proton transfer in synthesized 3,7-dihydroxy-2,8-di(4-methoxyphenyl)-4H,6H-pyrano[3,2-g]chromen-4,6-dion in polar and nonpolar solutions, films, and polycrystals at 293 and 77 K. In an excited singlet state, intramolecular proton transfer occurs in two stages. At the first stage, a tautomer with one transferred proton (OTP tautomer) is formed from the Franck-Condon state within ??₁ = 0.6 ps. At the second stage, the second proton is transferred within ??₂ = 3.1 ps and a tautomer with two transferred protons (TTP tautomer) is formed, which fluoresces in toluene at 293 K with a high quantum yield, ?? _f = 0.66, and the fluorescence spectrum of which is characterized by a large Stokes shift, 9900 cm^?1. At 293 K, polar solvents (dimethylformamide, dimethyl sulfoxide, ethanol, etc.) solvate the BFV molecule in the ground state, while, in the excited state, an OTP tautomer is mainly formed. In polar ethanol at 77 K, a dual fluorescence spectrum is observed, which is caused by the fluorescence emission of polysolvates with ?? _max ^f = 460 nm and TTP phototautomers at ?? _max ^f = 610 nm.     

A comparative study of the BaTiO3 film and the BaTiO3/(Ba0.7Sr0.3)TiO3 superlattice using X-ray diffraction and raman spectroscopy

A comparative study of the BaTiO₃ epitaxial film and the BaTiO₃/(Ba_0.7Sr_0.3)TiO₃ superlattice prepared by pulsed laser deposition on MgO(100) substrates has been performed. The complete parallel orientation of the film and the substrate has been established and the modulation period of the superlattice ?? = 28 nm, the parameters of the unit cell averaged over the period of the superlattice ??, and the unit cell parameters of the layers forming the superlattice have been determined using the X-ray diffractometry data. According to the X-ray diffraction data, the unit cell of the BaTiO₃ single-component film and the unit cell averaged over the period of the superlattice ?? are pseudocubic; however, an analysis of the polarization characteristics of the Raman spectra has suggested that the film and the superlattice have monoclinic symmetry (P _x = P _y ?? 0, P _z ?? 0). The transformation of the components of the soft mode due to the deformation of the epitaxial layers forming the superlattice has been discussed.     

Stabilization of high spin FCC Fe in (Fe/Pd)n multilayers

Polycrystalline (Fe/Pd)_n multilayers are grown onto sapphire substrates at room temperature in a UHV system. The number of periods n=40 and the thickness of Pd layers of t_Pd=4 nm are kept constant, whereas the thickness of the Fe layers is varied from 1.5 to 5 nm. Structural properties are studied by in situ reflection high energy diffraction (RHEED), scanning tunnelling microscopy (STM) and ex situ by X-ray diffraction at small angles and large angles. Analyzing the experimental data using the program SUPREX we obtain interplanar distances of d_Fe=2.03±0.01 Å for an Fe layer thickness larger than about 2.5 nm as expected for (1 1 0) planes of BCC Fe. For Fe layers with thicknesses less than about 2.5 nm the interplanar distance is d_Fe=2.1±0.01 Å, which is close to the distance between (1 1 1) planes of FCC Fe with a lattice parameter of a=3.64 Å. Magnetic susceptibility measurements at temperatures between 1.5 and 300 K for (Fe/Pd)_n multilayers with FCC Fe yield a magnetic moment per Fe atom of μ=2.7±0.1 μ_B, which is about 20% larger compared to μ=2.2 μ_B for BCC Fe. We show that the occurrence of the large magnetic moment originates from FCC Fe being in the high spin (HS) state rather than from polarization effects of Pd at Fe/Pd interfaces.     

Standing Wave Effects in Nuclear Resonance Bragg Reflectivity: Comparison of the Energy and Time Scales and First Experimental Results

The applicability of the concept of standing wave for the nuclear resonance Bragg reflectivity of synchrotron radiation has been tested for a microcrystalline (Fe/Cr)₂₆ multilayer. In the time domain the depth selectivity is strongly enhanced. A “scan” of hyperfine field distribution over bilayer depth was performed by a variation of the angle near the Bragg peak. In particular we observed that Fe/Cr and Cr/Fe interfaces are quite different and that the magnetic field orientation is different in the interfaces and in the central part of Fe layers. This revised version was published online in September 2006 with corrections to the Cover Date.     

Synthesis,magneto-optics and fluorescence of the ferromagnetic semiconductor Eu3(II)SiO5

Small single crystals of the transparent ferromagnet Eu₃SiO₅ have been synthesized by high temperature chemical transport. Magnetic measurements indicate T_c = 9°K and a saturation magnetization very close to 7 μ_B/Eu ion. Crystalline samples show a very low residual optical absorption and an absorption edge near 2 eV which displays a small red shift of 20 meV on cooling below T_c. Samples, containing a small percentage of dissolved EuO clusters, show in addition an absorption band at lower energies with a temperature dependence and magnetic behavior typical for EuO. The photoluminescence of the pure compound has a single emission band near 1·9 eV with a high quantum yield. At low temperatures also the fluorescence displays a red shift similar to that of the absorption edge. The fluorescence is accompanied with photoconductivity.     

Fluorescence of 5-hydroxy-6-methyl-(1-thietanyl-3)-pyrimidine-2,4(1 H,3 H)-dione at the transition with the second singlet-excited S 2 level to the ground level in acetonitrile solutions

The influence of the excitation wavelength on the fluorescence spectra of 5-hydroxy-6-methyl-(1-thietanyl-3)pyrimidine-2,4(1H,3H)-dione (TOMU) in acetonitrile solutions has been studied. It is found that, upon excitation of the singlet S ₂ state of TOMU luminescence, occurs from not only the first excited S ₁ level (??_max. = 350 nm, quantum yield ??(S ₁ ?? S ₀) = (4.5 ± 0.5) × 10^?3), but also at the transition from the second S ₂ level to the ground level (??_max = 305 nm, ??(S ₂ ?? S ₀) = (1.0 ± 0.1) × 10^?3).     

Photophysical parameters and fluorescence quenching of 7-diethylaminocoumarin (DEAC) laser dye

The optical properties including electronic absorption spectrum, emission spectrum, fluorescence quantum yield, and dipole moment of electronic transition of 7-diethylaminocoumarin (DEAC) laser dye have been measured in different solvents. Both electronic absorption and fluorescence spectra are red shifted as the polarity of the medium increases, indicating that the dipole moment of molecule increases on excitation. The fluorescence quantum yield of DEAC decreases as the polarity of solvent increases, a result of the role of solvent polarity in stabilization of the twisting of the intramolecular charge transfer (TICT) in excited state, which is a non-emissive state, as well as hydrogen bonding with the hetero-atom of dye. The emission spectrum of DEAC has also been measured in cationic (CTAC) and anionic (SDS) micelles, the intensity increases as the concentration of surfactant increases, and an abrupt change in emission intensity is observed at critical micelle concentration (CMC) of surfactant. 2×10⁻³ mol dm⁻³ of DEAC gives laser emission in the blue region on pumping with nitrogen laser (λ_ex=337.1 nm). The laser parameters such as tuning range, gain coefficient (α), emission cross section (σ_e), and half-life energy have been calculated in different solvents, namely acetone, dioxane , ethanol, and dimethyforamide (DMF). The photoreactivity of DEAC has been studied in CCl₄ at a wavelength of 366 nm. The values of photochemical yield (?_c) and rate constant (k) are determined. The interaction of organic acceptors such as picric acid (PA), tetracyanoethylene (TCNE), and 7,7,8,8-tetracynoquinonedimethane (TCNQ) with DEAC is also studied using fluorescence measurements in acetonitrile (CH₃CN); from fluorescence quenching study we assume the possible electron transfer from excited donor DEAC to organic acceptor forming non-emissive exciplex.     

Solid Fabry-Perot etalons for X-rays

Solid Fabry-Perot etalons for X-rays have been constructed using sputter deposition techniques, each etalon consisting of two Layered Synthetic Microstructures (LSM) Bragg diffraction structures separated by a carbon spacer. The individual LS mirrors contain fifteen tungsten layers (t_w = 8.5 Å) separated by carbon layers (t_c = 19.1 Å. The thick carbon spacers act as resonant cavities; for the structures reported on here the spacer thicknesses, t_sp, are 496.6 Å and 981 Å. The structures were characterized at grazing incidence in reflection using Cu Kα (λ = 1.5418 Å) radiation. The measured response of the etalons agrees well with calculation. Observed reflection efficiencies for Cu K_α were approximately 50 percent of that calculated. This discrepancy is believed to be the result of the interfacial roughness (～3.25 Å) between component layers and the sensitivity of the etalon response to the divergence of the incident X-ray beam.     

Interaction of physical fields in the acoustomagnetic effect

Experimental studies of the acoustomagnetic effect in a magnetic fluid are performed. The linear sizes of magnetic nanoparticles of the dispersed phase are determined by the acoustogranulometric method. The mean deviation of the diameters of magnetic nanoparticles obtained at eight fixed frequencies in the range of 18?C65 kHz from their average values ??d _max?? = 16 nm and ??d _max?? = 9 nm is 4.4%. These results are in satisfactory agreement with the results obtained by the magnetorelaxometry method for magnetite nanoparticles. On the basis of experimental data, in the framework of the concentrational model, interaction between the elastic and thermal fields and between the magnetic and dynamic demagnetizing fields in the acoustomagnetic effect is studied. The conclusions of the model theory are confirmed by the experimental results.     

Optical properties of cyclometalated Pd(II) and Rh(III) complexes based on 2-(4-biphenylyl)-6-phenylbenzoxazole luminophore with ethylenediamine

Using the methods of electronic absorption and emission spectroscopy, we have studied the optical properties of cyclometalated [Pd(C??N)En]CH₃COO and [Rh(C??N)₂En]Cl complexes of 2-(4-biphenylyl)-6-phenylbenzoxazole luminophore with ethylenediamine. We have shown that, along with a bathochromic shift of intraligand spin-allowed ??-??* optical transitions by 1000?C1800 cm^?1, complexes are characterized by the occurrence of long-wavelength bands of a mixed nature (intraligand-metal-ligand charge transfer) in the range of 369??392 nm and by competing intraligand fluorescence (419?C423 nm) and phosphorescence (511?C532 nm) processes under low-temperature (77 K) photoexcitation.     

Magnetoresistance anisotropy of strontium ruthenate films grown coherently on a TiO2-terminated SrTiO3(001) substrate

The SrRuO₃ films with a thickness of 80 nm have been coherently grown on a TiO₂-terminated SrTiO₃(001) substrate. Biaxial mechanical stresses induce a considerable difference between the unit cell parameters of the SrRuO₃ layer in the substrate plane (??3.904 ?) and along the normal to the substrate surface (??3.952 ?). The electrical resistivity of the SrRuO3 film decreases practically linearly with increasing magnetic field strength H when the latter is parallel to the current I _b and the projection of the easy magnetization axis in the substrate plane. At T = 4.2 K, ??₀ H = 14 T, and the magnetic field oriented along the hard magnetization axis, the negative anisotropic magnetoresistance of the grown layers reaches 16% and exerts a notice-able effect on the response of electrical resistivity of the SrRuO₃ film to the magnetic field.